Separations of gas mixtures containing nitrogen and oxygen are important industrial processes. The recovery of oxygen and/or nitrogen from air is practiced on a large scale. In the past, the primary method used for this separation was cryogenic distillation. More recently, pressure/vacuum swing adsorption (P/VSA) processes are being used in applications which have smaller gas requirements. In P/VSA processes, compressed gas is fed through a bed containing an adsorbent material with a preference for one of the components of the gas to produce an exit stream enriched in the other components. A stream enriched in the adsorbed component can be obtained by desorption.
P/VSA processes for selectively adsorbing nitrogen from gas mixtures, such as air, comprise contacting the gas mixture with a zone containing an adsorbent which is selective for the adsorption of nitrogen. Typically, the zone is operated through a series of steps comprising: adsorption, during which the gas mixture contacts the adsorbent, nitrogen is selectively adsorbed and oxygen passes through the zone and can be recovered as product; depressurization, during which the gas mixture contact is discontinued and the zone is reduced in pressure to desorb the nitrogen which can be recovered as product; and repressurization with air or oxygen product to the adsorption pressure.
The process performance depends on Bed Size Factor (BSF), 0.sub.2 Recovery, and Actual Cubic Feet evacuated/lbmol Evacuation gas (ACF/Evac). BSF (lb adsorbent/lbmole O.sub.2 in product) is an indication of the size of the adsorbent beds and the amount of adsorbent, the major impact of which is on capital equipment costs. Recovery is a measure of the O.sub.2 in the feed that is obtained as product. BSF is inversely proportional to the N.sub.2 working capacity, and inversely proportional to the O.sub.2 Recovery. Recovery in turn influences BSF and ACF, and has a strong impact on operating costs such as utility costs for the feed air blower. ACF/Evac, or actual cubic feet evacuated per lbmole of evacuation gas, influences capital (size of the vacuum train) and utility costs (power for running the vacuum pumps).
The use of zeolitic molecular sieves in PSA processes for air separation is well known. McRobbie in U.S. Pat. No. 3,140,931 claims the use of crystalline zeolitic molecular sieve material having apparent pore sizes of at least 4.6 Angstroms for separating oxygen-nitrogen mixtures at subambient temperatures. Of this group of zeolites, the Na form of X-zeolite (NaX) has often been used to advantage in air separation processes. There have been numerous efforts to develop improved adsorbent materials having high adsorptive capacity for N.sub.2 and high selectivity of N.sub.2 over O.sub.2. The Ca form of A-zeolite (CaA), for instance, was the basis of the Batta U.S. Pat. No. 3,636,679 for producing 90+% O.sub.2 from air via a PSA process. Later, Sircar and Zondlo (U.S. Pat. No. 4,013,429) patented a VSA air separation process using Na-mordenite (NaMOR). Coe et al. in U.S. Pat. Nos. 4,481,018 and 4,544,378 demonstrated the improved performance of faujasite compositions containing divalent cations, such as CaX, provided that they were activated in such a way that a preponderance of the polyvalent cations were in the dehydrated/dehydroxylated state.
Formed adsorbent particles containing zeolites used for equilibrium air separation also typically contain about 20 wt % inert inorganic material . The purpose of this material is to bind the zeolite crystallites into an agglomerate having high physical strength and attrition resistance in order that the zeolite crystallites can be used in adsorption processing. Those skilled in the art have generally believed that the addition of binder reduces the adsorptive properties of the adsorbent zone. In the past, the trend has been to try to reduce the levels of binder from the typical 20% to as low as possible, often as low as 5%, while at the same time maintaining adequate crush strength. For example, Heinze in U.S. Pat. No. 3,356,450 states that it is advantageous to obtain hard formed zeolite particles with the lowest possible binder content to maintain high adsorption capacity. He claims the use of a process which starts with molecular sieve granules bound with silicic acid, which are then treated with aqueous solutions containing alumina and alkali metal hydroxide, whereby the binder is converted to molecular sieve particles. The result is a practically binder-free (and therefore high capacity) shaped material with good abrasion resistance.
At the extreme of this trend toward reduced binder contents is the development of processes for preparing binderless bodies. Flank et al. (U.S. Pat. No. 4,818,508) teach the preparation of zeolites, particularly X, Y, and A, in massive bodies from calcined preforms made of controlled-particle-size kaolin-type clay. Kuznicki et al. (U. S. Pat. No. 4,603,040) teach the preparation of low silica X-zeolite (LSX) in the form of essentially binderless aggregates by reaction of calcined kaolin preforms in an aqueous solution of NaOH and KOH. W. R. Grace & Co. in GB 1,567,856 teaches a process for converting an extruded mixture of metakaolin and sodium hydroxide to A-zeolite. The advantage stated is that the method does not require the use of a binder such as clay, which usually reduces the activity of the molecular sieve by 15-20%.
Recently, adsorbents produced using these binderless bodies have been stated to have superior adsorptive properties when used for air separation. One such adsorbent is CaLSX, prepared by Coe et al., using the process of Kuznicki et al. (Coe, et al., "Molecularly Engineered, High-Performance Adsorbent: Self-Bound Low-Silica X Zeolite" in Perspectives in Molecular Sieve Science; Flank, W. H.; Whyte, Jr., T. E., Eds.; ACS Symposium Series 368; American Chemical Society: Washington, D.C., 1988; pp 478-491). "The self-bound LSX adsorbents do not have any binder to `dilute` the active component and lower the gas capacity." In addition, Coe et al. in U. S. Pat. No. 4,925,460 prepared chabazite from Y-zeolite extrudate. They state, "This method produces a superior adsorbent, since adsorptive capacity decreases as binder content increases." These materials were converted to the Li form and used for separation of air, among other gas separation processes. Thirdly, Chao in U.S. Pat. No. 4,859,217 claims a process for selectively adsorbing N.sub.2 using X-zeolite having a framework Si/Al molar ratio not greater than 1.5 and having at least 88 % of its AlO.sub.2 tetrahedral units associated with Li cations. He converted the bulk of the 20% binder in a zeolite "preform" agglomerate to X-zeolite crystals, obtaining essentially a binderless zeolite prior to ion exchanging into the Li form.
These more recent developments have shown really outstanding increases in capacity compared to the intrinsic capacity (i.e., capacity of the unbound zeolite) of adsorbents in the prior art. It is noteworthy that even the very high capacity materials described in these more recent developments were prepared in the binderless form. Thus, the prior art teaches that continued increase in capacity is better, there apparently being no upper limit. The desire for higher nitrogen capacity materials is understandable because it lowers the capital investment for the zeolite and adsorbent vessel. Higher nitrogen capacity also decreases the losses of O.sub.2 in the voids of the bed, which is expected to increase recovery and thereby lower power requirements.
However, nitrogen capacity is not the only property of the adsorbent that is important for low cost O.sub.2 production by P/VSA processes. The selective or preferential adsorption of N.sub.2 over O.sub.2 is also important, because any O.sub.2 which is coadsorbed on the adsorbent bed with N.sub.2 during the adsorption step is lost during the subsequent desorption step(s), resulting in lower O.sub.2 recovery. Selectivity (.alpha.) has conventionally been defined at a specific temperature and pressure in the following way: EQU .alpha.(N.sub.2 /O.sub.2)=(N.sub.N2 /Y.sub.N2)/(N.sub.O2 /Y.sub.O2)
where
N.sub.N2 =N.sub.2 coadsorbed at N.sub.2 partial pressure in the feed PA1 N.sub.O2 =O.sub.2 coadsorbed at O.sub.2 partial pressure in the feed PA1 Y.sub.N2 =mole fraction of N.sub.2 in the feed PA1 Y.sub.O2 =mole fraction of O.sub.2 in the feed
The very high nitrogen capacities of these recently developed adsorbents have generally been accompanied by higher selectivities. The prior art has recognized the benefits of this higher selectivity. Chao (above) points out the advantages of the high selectivity of the LiX materials and Coe et al. (above) point out the advantages of the high selectivity of CaLSX.
Selectivity and recovery impact power costs because they determine the amount of feed gas that must be compressed for the adsorption step per unit of product recovered. The cost of power is as important as the cost of capital in determining commercial viability of a PSA or VSA process. Thus, it is desirable to lower power consumption levels as much as possible.
The prior art VSA air separation processes using the recently developed very high nitrogen capacity materials described above have not been able to take full advantage of the very high selectivity of these materials in maximizing recovery and minimizing power requirements for compression. In contrast to the prior art, the present invention has found that for a given selectivity, nitrogen capacity lower than the very high capacity of these materials actually results in higher recovery. Furthermore, the evacuation step also requires high power consumption levels when these very high nitrogen capacity materials are used, so moderating their nitrogen capacity also results in evacuation power savings.
Thus, despite the previously recited substantial advances in adsorbents for PSA air separation of the prior art, there still exists a genuine need for more efficient air separation processes, particularly at very low power consumption levels, such as the present invention uniquely achieves as will be set forth below in greater detail below with regard to the present invention.